In conventional well drilling operations, surveying measurements are made both during and after drilling. The surveying measurements are used to determine both the orientation and absolute position of the well bore. The wellbore survey is often determined in part from measurements of the Earth's magnetic field in the wellbore. The measured magnetic field may be processed, for example, to determine a borehole azimuth (the direction of the well axis with respect to magnetic north).
Magnetic surveying measurements require that the Earth's magnetic field be known at the drilling location. The accuracy of the survey is limited by the accuracy of this “known” magnetic field. The Earth's magnetic field may be defined as a three dimensional vector, including a magnitude (field strength) and direction (defined, for example, by the magnetic declination and magnetic inclination of the field). Magnetic declination generally refers to the angle between magnetic north and true north. Magnetic inclination (also referred to as magnetic dip) generally refers to the angle of the magnetic field with respect a horizontal plane. The accuracy of the “known” field strength, magnetic declination, and magnetic inclination is directly related to survey accuracy with errors in these parameters resulting in corresponding wellbore orientation and placement errors. The placement errors can be significant (especially in deep wells) as they compound from one survey measurement to the next.
The magnetic field of the Earth (including the magnitude and direction components) is commonly obtained from previous geological survey data and/or global geomagnetic models in combination with suitable interpolation and/or mathematical modeling routines. The historical survey data and geomagnetic models tend to be limited in that they do not account for local magnetic field variations and solar magnetic activity. Solar activity in particular can be significant at high latitudes. Even at low latitudes (e.g., near the equator), the magnetic declination and magnetic inclination estimated using geomagnetic models can have errors approaching one degree. Such errors can result in significant borehole azimuth errors, for example, in a nearly horizontal, east-west wellbore, the magnetic azimuth error can be greater than four degrees. Moreover, these errors tend to be systematic (non-random) and can result in wellbore placement errors on the order of 100 feet or more in deep wells.
Measurement of the Earth's magnetic field at the surface while drilling is generally considered not to be practical or reliable, for example, due to local magnetic interference at the rig site. The oil industry has generally chosen to accept the aforementioned errors and the corresponding poor survey and well placement results. There is clearly a need in the art for obtaining an improved estimate of the Earth's magnetic field so as to facilitate improved magnetic surveying.